arcmap does not natively support pdf file format.  
you can ...  
 

 1. convert your pdf to an image file format. (like jpg, png, tif, etc)  
     I use [irfanview][1] with the [pdf plugin][2] to do this.
 2. purchase a third party product like [terrago][3] publisher.    
EDIT:  
I just noticed that you are probably wanting to insert as an image.  
 #1 or some derivative will do that.  
In fact file saveas in adobe will do that. (I have pro so not sure if it is in the free reader).   
![adobe save][4] 


  [1]: http://www.irfanview.com/
  [2]: http://www.irfanview.com/plugins.htm
  [3]: http://www.terragotech.com/products/geospatial-collaboration/terrago-publisher
  [4]: https://i.sstatic.net/Qxpct.png